Gas flow measurement, control and distribution has long been a critical subsystem in the manufacture of semiconductor devices. This subsystem, along with the RF power and the pressure control are the three major subsystems of a modern semiconductor fabrication tool. As the processes in the fabrication of integrated circuits, are chemical and/or physical reactions, the amount and specific chemistry present at the wafer surface defines the quality of the product. To have a repeatable quality outcome one needs repeatable and uniform chemistry over the wafer surface.
In addition, as smaller device geometries evolve in support of Moore's Law, the process time becomes shorter resulting in tighter tolerances. As such, the transition time for the changing of gas species delivered to the wafer becomes more important. Flow measurement and control farther from the wafer induces transport time, unknown resistances, and other issues that consume error budgets when changing from one species to another or from one gas mixture to another gas mixture.
FIG. 1 illustrates a gas handling system 100 of a typical modern semiconductor fabrication tool. Of note is the gas mass flow measurement and control function performed by thermal or pressure based MFCs (mass flow controllers) 111 located remote from a conventional showerhead 125 and wafer 126. Gas exiting a flow controlling gas box 110 typically travels through one or more feed conduits 199 to the process chamber 120 where it exhausts into one or more sealed spaces within the showerhead, collectively referred to as the plenum or plenums, and from there outward through a plurality of showerhead discharge holes.
The conventional showerhead 125 is shown in more detail in FIGS. 2A and 2B. As illustrated there, the conventional showerhead 125 typically contains more than 1,000 desirably identical small discharge holes in a pattern as shown in FIG. 2A and has cross section characteristics similar to those shown in FIG. 2B. The holes have moderate flow resistance so that uniform pressure builds up in the plenum due to its relatively large characteristic dimension resulting in its flow resistance being insignificant compared to that of the small discharge holes. The plenum pressure is on the order of magnitude of 10 torr at typical process flows for some tools. The discharge hole pattern shown in FIG. 2A is on a wafer side of the conventional showerhead 125 while FIG. 2B shows a discharge hole cross section. It is noted that a larger (e.g. 0.040″) drill is used for a portion of the discharge hole to allow easier manufacturing by reducing the length to diameter ratio of the drill. The majority of the pressure drop occurs through the smaller (e.g. 0.025″) drilled portion. Both drilled holes are small enough to create a dark zone that prevents RF power from creating a plasma and disassociating the gas molecules in the showerhead plenum prior to exiting the conventional showerhead 125.
Pressure based flow measurement and control has shown superior flow measurement and control compared to the earlier use of thermal based flow controllers. The pressure based devices rely on the concept that if the flow rate of a gas is characterized through a known flow path, as a function of the temperature and upstream and downstream pressures experienced by the flow path during a calibration process, this characterization can be used to measure and control gas flow in manufacturing processes by recreating the upstream pressures and measuring the current temperature and downstream pressure. Hardware for this gas control function is typically contained in a single device for each gas species utilizing a pressure based MFC or similar hardware on a gas stick in the gas box. MFCs are normally used to turn on, turn off, and control process gas flows at a desired flow rate.
However, commercially available MFCs are slow to transition between gases or to transition from higher flow rates to lower flow rates of a single gas. Of particular interest are low flow rate MFCs which experience slug flow delays associated with displacing gases in their branch legs before being carried to the process chamber by a higher flow carrier gas when their branch leg joins with the process gas header carrying all the gases. When the gasses first turn on the higher flow carrier gas pressurizes a section of the low flow gases branch. In addition, the pressurization for the process header requires a mass of gas that also introduces a pressurization delay. In addition, for some gases, NH3 and others, gas absorption or desorption to the tubing walls can affect the amount of a gas delivered to the chamber as molecules can attach themselves or release from the walls of the tubing carrying the gas. As such disposition of the gas mixture within the showerhead plenum is indeterminate, thus making for uncertainty about process conditions near the wafer.
Therefore, what is needed is a robust showerhead to integrate flow restriction for controlling mass flow of process gas to reduce transition times of the gas changes at the wafer, and improve control and knowledge of conditions near the wafer, in semiconductor tools in a manner that conserves error budgets.